Victims of the phone-hacking scandal were last night taking legal advice following the Guardian's revelations over News Group's secret £1m payout.

The football agent Sky Andrew said: "After being told certain individuals have taken legal action, I will take advice."

Speaking from Barcelona, on business, he said he was surprised by the apparent scale of the hacking. He suspected his phone had been tampered with when his pin number no longer worked. "When you are in an industry like mine, you suspect this type of thing could go on, but you don't actually expect it to happen to you."

Glenn Mulcaire, a private investigator working for News International, was jailed in 2007 for accessing Andrew's voicemails after a trial that also saw a former royal editor of the News of the World, Clive Goodman, jailed for hacking into the voicemails of royal aides.

But News Group has never publicly admitted any responsibility for Mulcaire's actions, which also saw the hacking of phones belonging to the model Elle Macpherson, the Liberal Democrat MP Simon Hughes, publicist Max Clifford, and the chief executive of the Professional Footballers' Association, Gordon Taylor.

Clifford said yesterday: "If all the allegations are true, then it is tremendously serious, because all of us were convinced by the police, by everybody, that this was just two people, a rogue journalist and a private investigator, and it was a one-off.

"But what is now coming out is an awful lot more damaging, not just for the News of the World, but also for the Metropolitan police, the Press Complaints Commission and, of course, for Andy Coulson. Am I taking legal advice? Yes. Have I decided what I am going to do? No."

He believed he became a target after falling out with the News of the World in 2005, when Coulson was editor, over its treatment of Kerry Katona. He told Newsnight he was called by his phone company: "I was told there were irregularities, they said 'we think your phone is being tapped'. The simple fact is that there are hundreds, maybe more, people whose phones have been allegedly tapped."

News of Taylor's successful action against the newspaper could open the floodgates for others eager to explore similar actions, Clifford said.

The TV and radio presenter Vanessa Feltz said she had called her solicitor immediately on discovering she was one of several celebrities who had been targeted by "blaggers" – private investigators who con BT, the DVLA, mobile phone companies and other organisations into handing over private details, then sell them to newspapers.

She said: "I am trawling through it all in my mind now, and the more I think about it, the more curious and bizarre the occasions where journalists suddenly appeared out of nowhere. In 2006, one of my daughters was taken to hospital. I remember journalists showing up that night at the hospital, and I was thinking: 'How could they possibly have known? I haven't told anybody yet.'

"It's one thing to see paparazzi at the Ivy. But I was finding them at Pizza Hut. There they were, even if it [the visit] had been arranged at the last minute."

Boris Johnson, the mayor of London and chairman of the Metropolitan Police Authority, was aware News of the World reporters had hacked into his phone before yesterday's story broke. A close source said he had been told by the police around the time of the Goodman trial that his phone might have been bugged.

Yesterday he said many good stories came from "contaminated sources" and that material was often obtained illicitly, – the issue was whether they were in the public interest.

Speaking on the BBC Today programme he said: "I'm sure the Met's commissioner will take account of whatever allegations have been made in the Guardian, but I am sure he will go ahead and take the proper action."